This invention relates generally to the field of spinal fixation devices used to secure, stabilize or fuse adjacent vertebrae in a rigid relationship. Such devices include for example disc prostheses, cages, implants, rods and plates. More particularly, the invention relates to the smaller subset of such spinal fixation devices consisting of a combination of a plate structure, which is adapted to be affixed externally to the adjacent vertebrae, and an insert spacer structure, such as an implant prosthesis or cage, which is adapted to be positioned between the adjacent vertebrae.
The spinal column provides vertical support and protects the spinal cord. The column comprises a plural number of vertebrae separated by discs, which cushion and dampen compressive forces. In many circumstances, whether due to injury, disease, degenerative causes or congenital defect, it is necessary to surgically remove all or a portion of an intervertebral disc. A spacer device must then be inserted in place of the removed disc. Typically, the ends of the adjacent vertebrae are shaved, drilled or cut to produce a particular surface configuration, and the space between the adjacent vertebrae is then filled with a prosthetic structure, such as an artificial disc or an implant structure, often referred to as a cage. The cage has an open interior and relatively open or porous upper and lower ends or walls which contact the vertebrae, whereby the open interior is filled with bone grafting and bone fusion material such that the bone growth between the vertebrae is promoted to result in fusion of the adjacent vertebrae. In addition or alternatively, the adjacent vertebrae are fixed in a rigid relationship to each other, either through the promotion of bone growth between the vertebrae, by fixation means incorporated in the implant structure or by external fixation structures attached to the vertebrae, such as plates or rods.
In some circumstances it has been discovered that a spinal or vertebral fixation device which incorporates in combination an internal cage or implant structure rigidly joined to an external fixation plate structure is the optimum device for correcting the problem. Such a device provides the structure to separate and position the adjacent vertebrae the proper distance and in the proper spatial relationship after the disc has been removed, provides a cage structure whereby fusion of the adjacent vertebrae by natural bone growth results in fixing and maintaining the vertebrae in a rigid relationship, and provides an external structure attached to the adjacent vertebrae to secure the vertebrae during the bone growth process and to provide additional fixation means in support of the bone fusion. Examples of such devices are disclosed in U.S. Pat. No. 4,599,086 to Doty, U.S. Pat. No. 4,955,908 to Frey et al., U.S. Pat. No. 4,892,545 to Day et al., U.S. Pat. No. 5,674,296 to Bryan et al., U.S. Pat. No. 5,888,223 to Bray, Jr., and U.S. Pat. No. 6,066,175 to Henderson et al.
Doty shows a spine stabilization device which comprises a generally solid cylindrical intervertebral disc insert having flat upper and lower ends. An external plate member is attached to the side of the disc insert, the plate extending beyond both ends of the disc insert, whereby the plate can be affixed laterally to the adjacent vertebrae using screws. Frey et al. shows an intervertebral prosthesis comprising a kidney-shaped, generally solid, disc insert having pairs of symmetrical plates, designated as fishplates, which extend upward and downward from one edge of the disc insert, the plates being provided with screw apertures for lateral attachment to the exterior of the adjacent vertebrae. Day et al. shows a device similar to the Doty device, but wherein the external plate and the intervertebral disc insert are formed as an integral unit. Bryan et al. shows a spinal disc prosthesis where the disc prosthesis is maintained in place by the combination of an annular member and two independent L-shaped supports, each support being attached to a vertebra. The disc prosthesis and the annular member are not secured to the vertebrae. Bray, Jr., shows an anterior stabilization device having a disc-shaped intervertebral spacer cage having a pair of superior lips and a single inferior lip, the lips provided with screw apertures for external fixation to the adjacent vertebrae. The inferior lip is centrally disposed relative to the superior lips, the superior lips being circumferentially spaced. Henderson et al. shows an intervertebral chamber or cage in combination with an external mounting plate. The mounting plate is provided with symmetrically disposed, equally sized screw apertures to receive mounting screws for affixing the plate to the adjacent vertebrae.
While the above referenced devices provide for an integral spinal fixation device having an intervertebral disc or cage and an external mounting plate, the particular structures and configurations shown present problems when affixing the devices to the spine of a patient. In particular, the known devices do not provide mechanisms to account for variations in the size, shape, separation distance and other vertebral factors, such that a surgeon is often forced to affix the devices in a less than optimum manner. For example, where multiple devices must be utilized to secure three or more adjacent vertebrae, the size and configuration of the plates may be too large, such that adjacent devices cannot be placed close enough together. It is an object of this invention therefore, to provide a spinal or vertebral fixation device which comprises the combination of an integral spacer cage member and external mounting plates, where the configuration of the insert cage and the mounting plates optimizes the fixation characteristics in order to provide for secure attachment and relative fixation of the adjacent vertebrae. It is a further object to provide such a device where the configuration of the plate members is such that multiple devices can be utilized on a single spinal column with the intervertebral cages disposed in relatively close proximity without having the plate members interfere and without reducing the efficacy of the mounting mechanism. It is a further object to provide such a device where the screw apertures in the mounting plates are of differing sizes and configurations, such that the surgeon has options to position the mounting screws in the most effective locations and alignments on the vertebrae, and where the screw apertures are beveled in a manner which causes compression of the vertebrae to the cage to enhance stability.
The invention is in general a spinal fixation device or intervertebral implant member which is adapted to be inserted between adjacent vertebrae in place of a natural intervertebral disc, such that the adjacent vertebrae are secured in a fixed manner to each other. The spinal implant member comprises in general a spacer member and fixation plate members, the spacer member comprising an apertured cage structure adapted to be inserted between the adjacent vertebrae and the fixation plate members having asymmetrical perimeters and comprising screw-receiving apertures adapted to be secured to each of the adjacent vertebrae.
The apertured cage structure of the spacer member comprises a main body, opposing vertebral contact surfaces on the superior and inferior ends of the main body, which are preferably generally planar and parallel to each other, a plurality of linear, parallel, beveled ridge members extending outwardly from each of the contact surfaces and oriented in the anterior-posterior direction, and an open interior, formed preferably for example by a pair of large openings or bores extending through the contact surfaces in the superior-inferior direction interconnected with a large lateral opening or bore extending through the main body. The contact surfaces are preferably rounded on the posterior portion of the lateral walls.
The fixation plate members are connected to the anterior side of the main body and extend in the superior-inferior direction. Each fixation plate member comprises a circular screw-receiving aperture and an elongated or slotted screw-receiving aperture. To accommodate the extended length of the slotted screw-receiving aperture, one side of each fixation plate member is of greater length than the other side, such that each fixation plate member is laterally asymmetrical. The slotted screw-receiving apertures are disposed on opposite sides of the device, which enables adjacent devices to be more closely arranged along the spinal column.